TRANSPORT MECHANISMS IN LIQUID MEMBRANES WITH ION-EXCHANGE CARRIERS

The extraction of lactate by an ion-pairing mechanism using a quaterna ry ammonium salt (Aliquat 336) was studied previously and a mathematic al model for equilibrium was developed. The evaluation of the equilibr ium constant (K-e) of the reaction between carrier and lactate allowed a good prediction of equilibrium for independent extraction and strip ping, within a large range of experimental conditions. However, when e xtraction and stripping operations are carried out simultaneously usin g liquid membranes and different concentrations of the feed and stripp ing solutes are used, the resulting osmotic pressure difference betwee n the two aqueous compartments (feed and stripping) has to be taken in to account for equilibrium prediction, if the membrane is not totally impermeant to water. Model prediction of equilibrium and identificatio n of the mechanisms involved on the transport of lactate and of the io n counter transported are presented. The influence of operating condit ions on each mechanism and their relative contribution to the overall transport are evaluated. Prediction of equilibrium is accurate in the absence of an initial osmotic pressure difference between the two aque ous compartments but exhibit a large deviation for increasing initial chloride concentrations. To counterbalance the initial osmotic pressur e difference an increasing hydrostatic pressure difference builds up l eading to salt transport across the liquid membrane.